Slip-on type pipe joints are one type of pipe joints. The slip-on type pipe joints are such that a spigot formed in an end portion of one of two pipes to be joined together is adapted to be inserted into a socket formed in an end portion of the other pipe and provided with a seal packing therein while compressing the seal packing. By virtue of this feature, it is possible to join the socket and the spigot together and to ensure sealing between the socket and the spigot by means of the seal packing.
A slip-on type pipe joint imparted with separation-preventive capability is known in which a locking ring is received in the locking ring receiving groove formed in the inner periphery of the socket and a projection is formed on the outer periphery of an end portion of the spigot to be inserted into the socket so as to engage the locking ring thereby preventing the spigot from being separated from the socket. In such a separation-preventive pipe joint, the locking ring is temporarily enlarged by the end portion of the spigot with its inclined surface serving as a guide upon insertion of the spigot into the socket, so that the projection can pass through the locking ring toward the deeper side of the socket.
In this type of separation-preventive pipe joint, the open end portion of the socket is formed as having a relatively large inner diameter to permit the projection on the end portion of the spigot to pass therethrough. Consequently, when the joining of the joint is completed, a clearance to a certain extent is formed between the inner periphery of the open end portion of the socket and the outer periphery of the spigot. This clearance enables the pipe joint to be imparted with a bendability between the socket and the spigot.
By providing a predetermined distance from the end of the spigot to the deeper end of the socket, when the projection is engaged with the locking ring and inserting the end of the spigot to a position slightly deeper than the location of the locking ring, it is possible to impart an expansibility between the socket and spigot.
Where a pipe line is to be constructed using pipe joints of this type, it is desirable that pipe joints for use in a straight portion of the pipe line be made expansible and bendable as above so as to accommodate themselves to external forces to be exerted thereon. In portions of the pipe line liable to receive nonuniform forces such as curved pipe portions, however, pipe joints subjected to expansion or bending cannot maintain a satisfactory joined condition.
In this case, particularly where a pipe joint is to provide for an interconnection between a straight pipe portion and a curved pipe portion, a socket of the normal type permitting expansion and bending is used as the socket for the straight pipe portion, so flint inconveniences may result when it is joined with the spigot for the curved pipe portion.
To eliminate such inconveniences, there is a proposal that a cylindrical sleeve is interposed between the end of the spigot and the deeper end of the socket in a pipe joint. The presence of the sleeve prevents the spigot from moving along the pipe axis, thereby preventing expansion and retraction of the pipe joint.
However, with mere provision of the sleeve, the problem of bending cannot be satisfactorily solved, though expansion and retraction of the pipe joint are prevented.